One solution is by researching techs which affect the few parameters of the ttrx file..

Anything else?

Easy

1. Research \ buy tech
2. Select some units of the same design which you wants upgrade
3. If upgrade exists (for selected design) you are able to press button and chose that upgrade do you want to apply (if available few).
4. Units move to reserve and after few days\week\months it ready.

I do not think that update of units design "id" of existing unit is very difficult.
Only one button which update design "id"

georgios wrote:And how this upgrade would be done?
One solution is by researching techs which affect the few parameters of the ttrx file..
Anything else?

Easy
1. Research \ buy tech
2. Select some units of the same design which you wants upgrade
3. If upgrade exists (for selected design) you are able to press button and chose that upgrade do you want to apply (if available few).
4. Units move to reserve and after few days\week\months it ready.
I do not think that update of units design "id" of existing unit is very difficult.
Only one button which update design "id"

Historically, in many cases there were multiple paths chosen - for example, an F-86F-35 might get replacement aircraft of F-100A, F-100C, F-84F Thunderjet.

The individual equipment as such should follow historical examples as far as possible and not just be a random picking of a future military unit, be it ship, air, or ground unit. Example - Upgrade : Cleveland class light cruiser - Upgrade to: Manchester
CL with 3"/50cal twin (replace the 40 & 20mm mounts] that served during Korean War.....and, Replacement Providence CLG or
Galveston CLG - but only a limited number of each of the missile cruiser conversion were undertaken. But, it is the most logical route if you are going to use a "Replace With" at all.

There have been a lot of errors in the past selection of appropriate Upgrade and Replace selections, and its takes some specific mechanics to include [not something a player can do].

The are still dozens of these in the SR-CW - generally left over from SR2020 & GC - and a further run through a large block of these is planned in the coming month (but it won't be for Update 3).